Among the many methods for enhancing the quality of an image, a very widely known one is histogram equalization. Using this technique, it is possible to enhance the contrast of a given image according to the occurences of different intensity levels contained within the image. It is also common to use this technique with respect to other image parameters like the brightness. In addition, histogram equalization may serve as a basis for image data reduction.
U.S. Pat. No. 5,388,168 discloses for example a picture quality improving circuit for enhancing the quality of an input signal. First, a cumulative histogram is derived from the input signal. Then, according to the level of the input signal, specific histogram values are selected based on which the input signal gets interpolated, thereby obtaining a corrected output signal.
Another example of a histogram equalization method is disclosed by U.S. Pat. No. 5,450,502. An input image is divided into a number of segments, for each of which a local histogram signal is generated. The local histogram signals are compared to a global histogram signal to obtain a comparison result. The comparison result is then used to further process the image, in particular to calculate a tone-reproduction curve which is used to correct the input image.